Re: [PATCH v5 3/4] tracing: Remove the backup instance automatically after read

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 28 Jan 2026 09:10:04 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> Since the backup instance is readonly, after reading all data
> via pipe, no data is left on the instance. Thus it can be
> removed safely after closing all files.
> This also removes it if user resets the ring buffer manually
> via 'trace' file.
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> ---
>  Changes in v4:
>    - Update description.
> ---
>  kernel/trace/trace.c |   64 +++++++++++++++++++++++++++++++++++++++++++++++++-
>  kernel/trace/trace.h |    6 +++++
>  2 files changed, 69 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index d39f6509c12a..7d615a74f915 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -590,6 +590,55 @@ void trace_set_ring_buffer_expanded(struct trace_array *tr)
>  	tr->ring_buffer_expanded = true;
>  }
>  
> +static int __remove_instance(struct trace_array *tr);
> +
> +static void trace_array_autoremove(struct work_struct *work)
> +{
> +	struct trace_array *tr = container_of(work, struct trace_array, autoremove_work);
> +
> +	guard(mutex)(&event_mutex);
> +	guard(mutex)(&trace_types_lock);
> +
> +	/*
> +	 * This can be fail if someone gets @tr before starting this
> +	 * function, but in that case, this will be kicked again when
> +	 * putting it. So we don't care the result.

			"So we don't care about the result."

> +	 */
> +	__remove_instance(tr);
> +}
> +
> +static struct workqueue_struct *autoremove_wq;
> +
> +static void trace_array_init_autoremove(struct trace_array *tr)
> +{
> +	INIT_WORK(&tr->autoremove_work, trace_array_autoremove);
> +}
> +
> +static void trace_array_kick_autoremove(struct trace_array *tr)
> +{
> +	if (!work_pending(&tr->autoremove_work) && autoremove_wq)
> +		queue_work(autoremove_wq, &tr->autoremove_work);
> +}
> +
> +static void trace_array_cancel_autoremove(struct trace_array *tr)
> +{
> +	if (work_pending(&tr->autoremove_work))
> +		cancel_work(&tr->autoremove_work);
> +}
> +
> +__init static int trace_array_init_autoremove_wq(void)
> +{

This isn't needed if there's no backup trace_array right?

Instead of creating a work queue when its not needed, just exit out if
there's no backup trace_array.

Oh, and the above functions should always test autoremove_wq for NULL.

> +	autoremove_wq = alloc_workqueue("tr_autoremove_wq",
> +					WQ_UNBOUND | WQ_HIGHPRI, 0);
> +	if (!autoremove_wq) {
> +		pr_err("Unable to allocate tr_autoremove_wq\n");
> +		return -ENOMEM;
> +	}
> +	return 0;
> +}
> +
> +late_initcall_sync(trace_array_init_autoremove_wq);
> +
>  LIST_HEAD(ftrace_trace_arrays);
>  
>  int trace_array_get(struct trace_array *this_tr)
> @@ -598,7 +647,7 @@ int trace_array_get(struct trace_array *this_tr)
>  
>  	guard(mutex)(&trace_types_lock);
>  	list_for_each_entry(tr, &ftrace_trace_arrays, list) {
> -		if (tr == this_tr) {
> +		if (tr == this_tr && !tr->free_on_close) {
>  			tr->ref++;
>  			return 0;
>  		}

Break the above into:

		if (tr == this_tr) {
			if (tr->free_on_close)
				break;
			tr->ref++;
			return 0;
		}

Why continue the loop if we found the trace_array but it's in the process
of closing?

-- Steve